In the construction of cooling systems for motor vehicles, a provision is often made to adapt the operation of the fan or its speed of rotation to the prevailing requirements for engine cooling. This provision is fulfilled by the installation of a controllable variable clutch. Preferred in the art is a fluid clutch in which the degree of fluid filling is variable under the control of mechanisms which sense the engine temperature or air temperature. The fluid clutch includes a housing which is rotatable with respect to the driving shaft and further includes a separating wall which defines a fluid reservoir and a drive chamber in which a drive disc is disposed. The separating wall has an opening whose size can be varied by a valve so as to control the amount of fluid in the clutch. In fluid clutches of this type employed in present-day service, the valve is actuated by bimetallic elements located at the front of the clutch. The change in shape of the thermal sensor is transmitted to the valve by a piston or an actuating pin.
The clutch on which the fan is mounted is located behind the radiator of the engine so that the temperature sensor lies in the stream of the cooling air whose temperature then becomes a measure of the coolant temperature.
A fluid friction coupling of this type is described for example in the German Auslegeschrift No. 12 84 186.
However, the known system described above is not adequate to fill the requirements of various modern cooling systems. For example, in some cases the engine is equipped with a separate radiator for cooling the air used for super-charging and this radiator is located in front of the main radiator for cooling engine water. In other constructions, the radiator is annular and employs a radial fan. Yet again, the fan may be disposed in front of the radiator or the engine may be air-cooled. In all these cases, the fluid friction clutch described above is not applicable.
This is the case because in the aforementioned constructions, the air is either additionally heated by passage through the secondary super-charger radiator or the air is not heated until it has passed the cooling fan. Accordingly, the variable fan clutch must be actuated on the basis of the coolant temperature by disposing a thermal switch or transducer at some critical location and transmitting its signal to the clutch. The medium for transmitting the signal may be compressed air, or electrical current. In the first of these two cases, the necessary piping and seals are a substantial additional expense. A pneumatic control system of this type is known for example from U.S. Pat. No. 3,880,265.
The electrical signal transmission to the clutch is substantially simpler and a control system of this type for use with fluid clutches is known for example from the German Pat. No. 1,270,339. This patent corresponds to U.S. Pat. No. 3,250,355. In this known system, the clutch is actuated by a solenoid which is relatively expensive and heavy enough to apply additional loads to the clutch bearing as well as to the bearing on the driving side, i.e., the water pump bearing or crankshaft bearing.